1. Field of the Invention
This invention relates to an electroacoustic transducer device for converting an electrical signal into sound, including subsonic or ultrasonic sound, in a gas such as air or in a liquid such as water. More particularly, the invention relates to the combination of a driver and a structural radiation aide for an electroacoustic transducer device. Still more particularly, the invention relates to an electroacoustic transducer device in which the driver is in the form of a plurality of piezoelectric driver elements and the structural radiation aide is a flexible panel.
2. Description of Related Art
A piezoelectric material is a material which, upon application of an electrical voltage, converts the voltage into a mechanical vibration or, conversely, converts mechanical vibrations into electrical signals. Consequently, piezoelectric materials have long been used in electroacoustic receivers and transmitters.
A drawback of using piezoelectric materials in electroacoustic transducer devices is that such devices generally perform much better at high frequencies than at low frequencies because, although very large forces can be produced by applying electric fields to the constrained piezoelectric crystals or ceramics used as driver elements in electroacoustic transducer devices, the accompanying strain is relatively small. In other words, it is difficult to get the large vibrational amplitudes needed for high intensity, low frequency sound using piezoelectric devices. In addition, the mechanical impedances of piezoelectric materials are generally closest to those of liquids and solids, rather than gases, thus limiting the energy transfer efficiency of piezoelectric electroacoustic devices designed for use in air, such as loudspeakers.
A number of ingenious schemes have been devised for introducing a mechanical advantage that would reduce the driving force in exchange for increasing the force distance of a piezoelectric driver element. One such scheme is to provide the commercially available device 1 illustrated in FIGS. 1 and 2. The device 1 includes a thin layer of piezoelectric ceramic material 2 bonded to a thin conductive metal disc 3, made for example of brass, bonded via a flexible adhesive layer 7 to a conductive metal ring 6, also made of brass. When an electric voltage is applied between the surfaces of the thin ceramic disc, the disc varies in thickness, and also in radius. The relatively large forces of expansion and contraction produced are transferred to the surface of the metal disc 3 to which the ceramic is bonded, with the effect that the disc bows up in the center when the ceramic disc expands, and down when the ceramic disc contracts.
Performance is improved by loosely bonding a second conductive metal disc 5, including a second piezoelectric layer 4, to the metal ring 6 to form a capsule 1. When the two ceramic layers are subjected to attenuating electrical fields, the thickness of the capsule varies with the frequency of the applied voltage, thereby changing the electrical signal into a radiated soundwave.
While the low frequency performance of the above-described transducer, also known as a transflexural piezoelectric element, is greatly improved relative to other conventional piezoelectric drive arrangements, the intensity of sound which can be radiated is nevertheless limited, and the directionality of the sound cannot be controlled. In addition, the radiating area possible with such transducers is relatively small, the bandwidth is relatively narrow and, in the case of ordinary loudspeakers, expensive and difficult-to-design enclosures are needed in order to eliminate the effect of cancellation between positive and negative pressures which occurs at low frequencies because the wavelengths generated are greater than the size of the enclosure. Further, the problem of impedance mismatches makes this type of element unsuitable for use in air. The specific driver shown in FIG. 1, for example, has previously been used only as a single element for sensing and generating underwater vibrations.
In an extension of the well-known concept of placing a driver in a paper speaker cone for purposes of controlling the mechanical impedance and frequency range of an audio loudspeaker element, it has also previously been proposed to arrange a plurality of piezoelectric elements between two opposed rigid plane resin foam plates, each having multiple recesses, each of the recesses accommodating a piezoelectric driver. The recesses are bigger than the drivers and extensions of the plate are provided for supporting the substantial centers of the drivers.
The plane resin foam plate type of piezoelectric speaker structure is disclosed in U.S. Pat. Nos. 4,969,197 and 5,031,222, both to Takaya. The intent of the two patents is to provide a device in which the two rigid resin foam plates forming the speaker diaphragm or radiation aide are vibrated by driving the piezoelectric drivers, causing sound to be emitted from both main surfaces of the diaphragm. Significantly, each piezoelectric driver is in the form of a single metal disc with a piezoelectric material bonded to it and which is contained in the space of the diaphragm without being contacted by any other element, except for the center supports. Consequently, the rigid resin foam plates do not restrict vibration of the edges of the piezoelectric driver.
As is best shown in FIG. 1 of U.S. Pat. No. 4,969,197, the multiple driver piezoelectric speaker structure of Takaya was intended to be an improvement over prior art structures in which the piezoelectric driver is completely enclosed in foam. The completely enclosed driver was believed to be impractical because the vibration of the piezoelectric driver element, which is simply a vibrating film on which the piezoelectric elements are attached, cannot overcome the resistance of the foam plate in order to provide sufficient output intensity.
Therefore, although it has previously been proposed to enclose piezoelectric driver elements completely within a foam element to form a diaphragm or sounding board, the approach was found to be unsatisfactory because the foam overly restricted the vibration of the conventional piezoelectric element. The present invention makes it possible to overcome this problem and provide a piezoelectric panel in which recesses of the type disclosed by Takaya are unnecessary by utilizing a different type of piezoelectric driver element, shown in FIGS. 1 and 2, which had previously not been considered for use in panel arrangements.
In addition, the previous foam plate diaphragm structures suffered from the limitations that foam is porous, and therefore not suitable for use in liquids and corrosive environments, and that the foam used could not easily be shaped or bent for use in restricted spaces. The present invention overcomes these limitations by potting the driver elements in a material which is both non-porous and flexible.